This makes life easier when calling e.g. `saul_reg_write()` with data
stored in flash.
As now the signatures for reading and writing differ (in that `const`
qualifier only), `saul_notsup()` is split into `saul_write_notsup()`
and `saul_read_notsup()`. However, one is implemented as a symbol alias
of the other, so that ROM consumption remains unchanged.
A memset was used to clear two of the SAUL values in phydat_t::val
The change is to replace that by the following
res->val[1] = 0;
res->val[2] = 0;
which makes the code more obvious.
